Amyloidosis refers to a variety of conditions in which amyloid proteins are abnormally deposited in organs and/or tissues. These amyloid proteins sometimes exist in an abnormal fibre-like form, called amyloid fibrils or amyloid deposits, that build up and progressively interfere with the structure and function of affected organs throughout the body. Different proteins are implicated in different types of amyloid disease, and treatment depends on the particular amyloid protein.
Transthyretin-associated amyloidosis is a general denomination for a group of amyloid diseases that are specifically associated to transthyretin abnormal misfolding, aggregation (fibril formation) and subsequent deposition. Transthyretin (TTR) protein is a serum and cerebrospinal fluid carrier of the thyroid hormone thyroxine and retinol. Mutations in the TTR gene, which is located on human chromosome 18q12.1-11.2, sometimes result in a destabilization of the TTR protein, leading to abnormal aggregation and transthyretin-associated amyloid disease. More than 80 amyloid forming variants of TTR are known, of which the most frequent is called TTR V30M.
Familial amyloid polyneuropathy (FAP), also called transthyretin-associated hereditary amyloidosis, transthyretin amyloidosis or Corino de Andrade's disease, is an autosomal dominant neurodegenerative disease. Usually manifesting itself between 20 and 40 years of age, it is characterized by pain, paresthesia, muscular weakness and autonomic dysfunction. In its terminal state, the kidneys and the heart are affected. FAP is characterized by the systemic deposition of amyloid variants of the TTR protein, especially in the peripheral nervous system, causing a progressive sensory and motorial polyneuropathy. This disease is by far the most common type of hereditary amyloidosis in the world.
Other types of transthyretin-associated amyloidosis are familial amyloid cardiomyopathy and senile systemic amyloidosis, caused by the deposition of amyloid TTR in the heart, and leptomeningeal amyloidosis, where amyloid deposits of TTR are found in the walls of leptomeningeal vessels, in pia-arachnoid, and also in subpial space deposits. The latter condition is associated with a clinical picture of central nervous system impairment manifest as dementia, ataxia, and spasticity.
Whilst there is, as yet, no treatment that blocks amyloid deposition or speeds up its removal, treatment of amyloid diseases is aimed at supporting the function of failing organs. Liver transplantation has been often used as a treatment for transthyretin-associated amyloidosis, particularly FAP, since TTR protein is mainly produced in the liver. Replacement of the liver containing a mutant TTR gene by a liver that makes normal transthyretin protein is aimed at preventing the formation of further amyloid and can stabilise the disease. Liver transplantation has been performed in patients with FAP, with great success in many cases. However, a liver transplantation is not always an available option and, besides, as experience increases, it is becoming clear that liver transplantation for FAP should take place before too much damage to the nerves or heart has already occurred. Sadly, the latter may occur without causing any symptoms.
Very few compounds have been described as exerting an inhibitory activity against fibril formation and subsequent deposition of TTR. Among these, iododiflunisal has been reported as a potent amyloid inhibitor in vitro by Gales et al (Gales L, Macedo-Ribeiro S, Arsequell G, Valencia G, Saraiva M J, Damas A M. “Human transthyretin in complex with iododiflunisal: structural features associated with a potent amyloid inhibitor”. Biochem J, 2005, vol. 388, p. 615-621). Further, patent application WO 2005/113523 discloses benzoxazole compounds for stabilizing TTR amyloid protein, thus preventing the formation of TTR amyloid fibrils. These compounds are claimed as useful for the treatment of transthyretin-associated amyloid diseases.
Particularly, a benzoxazole derivative called tafamidis (2-(3,5-dichlorophenyl)-1,3-benzoxazole-6-carboxylic acid has been described to inhibit TTR abnormal aggregation and fibril formation, and is undergoing clinical trials for the treatment of FAP. Despite being a promising perspective, tafamidis is still under evaluation in the main official medicine registration offices. Thus, the clinical relevance of tafamidis still needs to be clarified.
Despite the considerable effort that has been made in the field, there is to date no effective pharmacological therapy for the treatment of FAP. It is therefore desirable to provide alternative compounds for the treatment of FAP and other transthyretin-associated amyloidosis.